Cathode sputtering refers to a process which involves the coating of a wafer mounted within a processing chamber. An applied electric field negatively biases a target of the material to be sputtered with respect to the processing chamber and wafer. A backplate may support the target. Upon introduction of an inert gas into the chamber at low pressure, the applied electric field ionizes the gas. As a result, positive ions from the gas bombard the target to cause sputtering of the target material onto the wafer. A magnet or electromagnet may be located behind the target to provide a magnetic field which confines the ionized "plasma" adjacent to the target. The magnetic field increases the efficiency of the sputtering process.
Some typical metals used in sputtering include aluminum alloys, gold, silver, copper, titanium, platinum, and refractory metals. Due primarily to the high purity required for sputtering, the cost of these sputtering materials is generally quite expensive, sometimes exceeding thousands of dollars per pound of material. For this reason, maximum target material utilization is a major consideration in the design of sputtering equipment. Another important consideration relates to the effective coverage of features on the surface of the wafer, either by planarizing an uneven surface or by adding additional conformal layers. Some sputtering equipment employs a variation of the magnetic field, or multiple erosion zones, or both, to coat wafer features in the desired manner.
In addition to maximum target utilization and effective coverage of a wafer surface, other practical considerations such as target mounting and costs of manufacture also play an important role in the design of sputtering equipment. While some target configurations may provide excellent performance, these advantages may be offset by the difficulty or costs associated with properly mounting the target to a backplate or the cathode assembly. For the most part, the machining of targets is difficult, expensive and time consuming.
Thus, in depositing thin films by sputtering, the costs and operating lifetime of the sputtering target are significant factors in the cost of the coated product. This is particularly true in the semiconductor industry where even targets of common materials are so highly purified and characterized as to have high intrinsic material value.
Additional factors which contribute to the cost of use include complex machining which is only utilized over the short life of a target; deeply contoured target shapes which require removal of large amounts of expensive material as machining scrap; and the utilization of the target, or the quantity of final products which can be coated before depletion of the target.
A cost effective target, therefore, is simple to machine or fabricate with minimal generation of scrap, and yields a large quantity of product over the lifetime of the target.
A typical sputtering target for a cathode assembly includes a disk shaped portion having a front sputtering face and a rear face opposite the sputtering face, and a hub which projects from the rear face of the disk and which is adapted to be secured to the cathode assembly. The hub is either integral with the disk, as when the hub and disk are machined or forged or pressed (as used herein, the term "forged" encompasses "forged" and "pressed") from a single piece of target grade material, or the hub is fabricated as a separate piece and is secured to the rear face of the disk portion by fasteners, for example bolts or the like. It will be appreciated that machining a disk with the integral hub out of a single flat piece of target grade material creates a large amount of machining scrap, and in addition requires a great amount of costly machining. If the hub is fabricated from a separate piece of material and is attached to the rear face of the disk portion of the target with fasteners, the fasteners, which penetrate into the disk from the rear face, limit the amount of the disk which is usable for the sputtering process in the vicinity of the interconnection of the hub to the disk. That is to say, as the sputtering face and target material erode during use to define a final sputtered face contour and a residual target thickness t measured from the rear face in the hub/disk interconnection region, the target cannot be sputtered to the extent that the fastening elements, for example, bolts, would begin to appear through the final sputtered face contour; i.e., the residual target thickness t measured from the rear face is limited by, and must be greater than, the depth the fastener elements protrude into the disk from the rear face.
However, the length of the fastener elements which extend into the disk from the rear face thereof and hence the depth of the tapped holes in the disk which receive those fastener elements cannot be shortened to the extent that the structural integrity of the hub/disk interconnection during sputtering is jeopardized for the sake of minimizing the residual target thickness t in order to maximize the amount of sputterable target grade material. For example, current hub/disk assemblies which utilize bolts as the fastening means of hub to disk generally require tapped holes for receiving the bolts at least 0.375 inch deep when the disk material is soft titanium or aluminum to ensure the structural integrity of the hub/disk joint during sputtering. Thus, current hub/disk assemblies fail to utilize, or otherwise waste, 0.375 inch of target grade material in the region of the hub/disk interconnection.
Should the target be forged from a starting slug of target grade material in order to form an integral hub/disk target assembly, still different problems are posed. During forging, flow lines are developed in the displaced target material. These flow lines can change the crystal orientation and hence the sputtering properties of the target near the center of the target where the hub is formed. When progressing from the front face of the target to the back face, flow lines which are generally parallel the back face are first encountered, but as sputtering continues, flow lines are encountered which turn upwardly from the disk portion of the target into the hub. The crystal structure of the target material in this region of upturned flow lines is different than that in the region of parallel flow lines and is generally less desirable as the sputtering properties of the material there are inferior to that of the material in the area of parallel flow lines. In current forged targets the residual target thickness t measured from the rear face of the target disk which includes the upturned flow lines and undesirable sputtering properties is at least 0.375 inch for all target grade materials, and for some current forged targets the undesirable upturned flow lines extend all the way to the front face of the target. Thus, assuming that current forged targets are not sputtered into the undesirable upturned flow lines, those targets fail to utilize, or otherwise waste, 0.375 inch of target material for all types of target materials in the region of the hub/disk interconnection, and for some forged targets, the targets exhibit the undesirable upturned flow lines all the way out to the front face resulting in inferior sputtering qualities throughout the entire thickness of the target.
It is therefore an objective of the present invention to provide a sputtering target which reduces the labor and amount of scrap normally attendant the machining of a target having an integral hub from a single piece of material.
It is another objective of the present invention to provide a target which utilizes a minimum thickness of the target material to attach the hub to the target yet which does not sacrifice the structural integrity of the hub/disk joint in the process.
It is yet another objective of the present invention to provide a forged target having superior sputtering properties near the center of the target in the location of the hub.
It is still another objective of the present invention to provide a forged target having upturned flow lines which occur nearer the back face of the target than those of current forged targets to thereby increase the utilization of the target grade material.